The present invention relates to a seat cushion used for various kinds of vehicles such as automobiles and rolling stock, and a process of producing the seat cushion.
To improve a feeling of seating and a vibration transmission characteristic of a seat cushion, there has been proposed an attempt in which a slab of flexible polyurethane foam having a specific size is integrally joined on a seat surface portion of a seat cushion main body formed of a polyurethane molded foam by a post-sticking process or an integrally foaming process. In particular, to omit the post-treatment step, the integrally foaming process has been widely adopted.
In the integrally foaming process, a foamable liquid for forming a polyurethane molded foam partially permeates in a slab flexible polyurethane foam. In this method, a foamable liquid impregnated surface of the slab foam is hardened by foaming of the foamable liquid, to form a hardened layer. In design of a seat, such a hardened layer is sometimes considered undesirable for the reason that it brings a feeling of physical disorder, and in other cases, it is positively incorporated into the design for the reason that it brings a feeling of surface rigidity.
The above process of joining a slab flexible polyurethane foam on a polyurethane molded foam by integral foaming, however, has the following disadvantages:
(1) The surface of the slab flexible polyurethane foam, which is brought in contact with a foamable liquid for forming the polyurethane molded foam, is hardened by foaming of the foamable liquid impregnated therein, so that a hardened layer with a surface rigidity is formed. However, in this case, it is impossible to adjust the hardness and thickness of the impregnated/hardened layer exerting a large effect on the riding comfortability of the seat cushion. PA1 (2) When the foamable liquid for forming the polyurethane molded foam is foamed, the foamable liquid having impregnated in the slab flexible polyurethane foam is expanded by foaming, and thereby the contact surface of the slab foam with the foamable liquid is also expanded, to be thus extended.
As a result, there occurs a difference in size between the contact surface (interface) of the slab foam having impregnated with the foamable liquid and the opposed surface thereof having not impregnated with the foamable liquid. This causes a phenomenon shown in FIG. 2, in which there occur local depressions in the slab foam on the front surface side of the product, that is, a camber and/or a shrinkage of the seat.